This invention relates to internal combustion engines and more particularly to an internal combustion engine mounting a novel exhaust emission control device.
Ever-increasing motor-vehicle traffic has given rise to the problem of air pollution by the exhaust gases of internal combustion engines. Various proposals have been made to solve this problem by providing various devices for cleaning the exhaust gases of the combustion engines by reducing the noxious components thereof. In the case of internal combustion engines, particularly four-stroke cycle engines, for motor vehicles, the most commonly adopted device is one which uses an exhaust manifold or a manifold thermal reactor similar to the exhaust manifold. Generally, in ordinary internal combustion engines, the fuel-air mixture drawn by suction into the engines has a theoretical air-fuel ratio and sometimes the mixture may become richer. This makes it necessary to supply excess secondary air for recombustion of exhaust gases into the exhaust port or manifold for the thermal reactor in order to promote oxidation of non-combusted hydrocarbons and carbon monoxide. The secondary air is usually supplied under pressure by means of an air pump of the vane type. Disadvantages associated with this system are that the operation of the pump has loss power and causes noise. Moreover, the air pump is low in durability.
In view of these disadvantages of supplying secondary air by means of an air pump, proposals have been made in recent years to use a system wherein a rich fuel-air mixture is supplied to a cylinder or a group of cylinders and a lean fuel-air mixture is supplied to another cylinder or another set of cylinders, and exhaust gases vented from the two cylinders or two sets of cylinders are introduced into a manifold thermal reactor, so that the exhaust gases discharged from the cylinder or cylinders for the lean fuel-air mixture are utilized as secondary air so as to do without an air pump. Additional advantage of this system lies in the fact that, since the fuel-air mixtures combusted in the cylinders are either richer or leaner than a fuel-air mixture of the theoretical air-fuel ratio, it is possible to reduce the amount of nitrogen oxides in the exhaust gases of combustion engines. However, some disadvantages are associated with this system. The need to use two cylinders as a single unit would cause unbalanced combustion between the two cylinders, fouling of the ignition plug, and distorsion of the cylinders due to differences in temperature. Moreover, even in the case of a motorcycle of compact size and light weight, the engine must have two cylinders which renders the construction of the engine complex.